This is the description (as per error files) of the error code that i get.

ORA-02231 missing or invalid option to ALTER DATABASE
Cause: An option other than ADD, DROP, RENAME, ARCHIVELOG,
NOARCHIVELOG, MOUNT, DISMOUNT, OPEN, or CLOSE is specified in the
statement.
Action: Specify only legal options.

SQL> alter database set time_zone='+04:30';
alter database set time_zone='+04:30'
*
ERROR at line 1:
ORA-30079: cannot alter database timezone when database has TIMESTAMP WITH LOCAL TIME ZONE columns

Quote:

ORA-30079: cannot alter database timezone when database has TIMESTAMP WITH LOCAL TIME ZONE columns
Cause: An attempt was made to alter database timezone with TIMESTAMP WITH LOCAL TIME ZONE column in the database.
Action: Either do not alter database timezone or first drop all the TIMESTAMP WITH LOCAL TIME ZONE columns.

Depending if you want to keep or not the current data in these columns.
For myself, I export the tables with these datatype, drop the tables, alter database, import the table and update the data to reflect the change of database timezone.